Gravity surveying is one technique in modern exploration for mineral and petroleum commodities. For example, detection of geophysically significant subsurface anomalies potentially associated with ore bodies or hydrocarbon deposits can be made using gravity surveying techniques since the existence of gravitational anomalies usually depends upon the presence of an excess or deficit mass associated with the deposit. For example, the gravitational anomaly of a body of ore with a density contrast of 300 kg m−3 and a dimension of 200 m buried at a depth of 100 m is typically 2×10−6 ms−2, for example, which is 0.00002% of the normal Earth gravity field. This relatively small effect is normally measured in units of milli gals (mGal), which is the unit for the free air and Bouguer gravity field measurements and is equivalent to 10−5 m/ss. Thus, for the above example, the body of ore would be represented by 200 mGal.
Currently, many measurements have been made using instruments of the LaCoste/Romberg type that are essentially ultrasensitive spring balances detecting a small difference in weight caused by the gravity anomaly. The measurements are subject to a wide variety of environmental influences, and measurements should be performed relative to a standard point that is used regularly during the survey as a fixed reference for removal of drifts in the instrument. This procedure can be slow, and may require extensive information on local topography and geology since a normal variation of gravity with height is approximately 0.3 mGal per meter. Within moving platforms, such as aircraft, using this type of relative gravity instrument can be difficult because using precision radar altimeters and pressure sensors to achieve vertical position to as little as one meter can impose limitations on the order of a few hundred mGals on the gravity data.
For this reason, some large scale geophysical prospecting has progressed towards gradiometry. In principle, measurement of a gradient of a gravity field over a known baseline allows accelerations due to motion of the platform itself to be cancelled out. Gravity gradients are the spatial derivative of the gravity field, and have units of mGal over distance such as mGal/m. The standard unit of gravity gradiometry is the Eötvös (E), which is equal to 10−9/s2 or a tenth of a mGal over a kilometer (e.g., gradient signatures of shallow Texas salt domes are typically 50–100 E).
Three-dimensional Full Tensor Gradient (3D FTG) technology was developed by the US Navy and later adapted to the Oil & Gas industry to complement seismic technology and provide an independent method of imaging around salt and basalt areas, for example. Full tensor gradiometry measures the gradient of the gravity field, and thus is measuring a small quantity (1 Eotvos=0.1 mgal/Km). This can require unique acquisition parameters to be used to insure a high quality survey.
Acquisition of 3D FTG data requires the operator to consider the unique nature of such a high frequency, small amplitude measurement. Acquisition parameters are dictated by several factors including, for example, water depth, target depth, geologic concerns and the type of imaging problem being modeled. FTG data may be recorded in 800 MB files (about 2 hours of data) for archival and quality assurance checks, for example.
The interpretation of geophysical data collected from airborne measurements occurs on the ground in a geological office. The purpose of the interpretation is to establish priorities for subsequent investigation. Thus, data can be sent via satellite to the Bell Geospace Technical Department in Houston for analysis, for example. An operator then processes each data file to determine noise values, acceleration values, platform performance, individual instrument data plots, environmental conditions, positional accuracy, and other criteria, for example. In the event that a survey line of data is substandard, the line is identified and will need to be repeated to ensure optimum quality of data, and thus a crew will need to be sent back out to the survey area.
However, to insure an efficient geologic survey, it would be desirable to monitor data in close to real time to determine whether signal to noise requirements and other acquisition parameters are being met to lessen the inconvenience of repeating survey lines.